1. Field of the Invention
The present invention relates to an image pickup apparatus, and particularly to an image pickup apparatus having a focus detection function.
2. Description of the Related Art
In image pickup apparatuses such as digital cameras and the like, a technique to perform focus detection according to a pixel signal outputted from an image pickup device has been proposed. Herein, focus detection refers to, in an image pickup device, detecting a focus position (focusing condition) of an optical imaging system with respect to an object. To be specific, there is proposed a focus detection method called a phase difference detection scheme which performs focus detection according to a phase difference between pixel signals outputted from a plurality of pixels for focus detection, in an arrangement in which pixels for photographing that receive a light flux that has passed through an optical imaging system and output pixel signals, and a plurality of pixels for focus detection that receive only a light flux that has passed through a different pupil region of the optical imaging system and output pixel signals are provided in an image pickup device, respectively.
However, in focus detection by using a phase difference detection scheme, for example, when the object has a low illuminance, and when a large defocus has occurred, the signal level obtained from the focus detection pixel will decline. Moreover, since the S/N ratio will decline in accordance with such a decline of the signal level, a problem will arise in that when a correlation computation is performed simply by performing a gain-up processing, an error will occur in the computation result thereby disabling the performance of suitable focus detection.
On the other hand, for the purpose of avoiding the above described problem, for example, Japanese Patent Application Laid-Open Publication No. 2009-3122 discloses an image pickup apparatus for performing focus detection by using the above described phase-difference detection scheme, which is configured to perform a correlation computation for determining a defocus quantity (focus deviation quantity) on the output signals from pixels for focus detection by using synthesized signals obtained by adding and synthesizing output signals from focus detection pixels which are present in the surroundings of the pixels for focus detection.